Method of manufacturing a dental prosthesis

ABSTRACT

A process for manufacturing a non-metallic dental prosthesis including a ceramic frame and an esthetic ceramic veneer including the steps of: preparing the ceramic frame by press molding a composition comprising from 1 to 50 parts by weight glass particles and from 50 to 99 parts by weight ceramic particles at a molding temperature from 800 to 1300° C.; applying a slurry comprising a dental ceramic veneering composition to the thus prepared ceramic frame; and firing the ceramic frame having the slurry applied thereto at a temperature lower than the molding temperature to produce said dental prosthesis. The dental ceramic veneering composition may have a melting point at least 50° C. less than the molding temperature.

This application is a division of application Ser. No. 08/846,465, filedMay 1, 1997; U.S. Pat. No. 5,849,068, which is a continuation of patentapplication Ser. No. 08/653,517 filed May 23, 1996 abandoned, which is acontinuation of patent application Ser. No. 08/423,056 abandoned, filedApr. 17, 1995, which is a continuation of patent application Ser. No.08/083,137 filed Jun. 24, 1993, abandoned.

The invention relates to a dental prosthesis. In particular, theinvention provides dental prostheses of high strength. Dental prosthesesprepared according to the present invention include caps, crowns,bridges, veneers, inlays and onlays, for example, peripheral caps andcrowns, bridges that are placed on stumps of natural teeth to supportsimultaneously the remaining parts of at least two teeth by compensatingultimately for one or more missing teeth. To produce supporting metalstructure parts for dental prostheses, such as caps and frames, metal isused for its high strength, but for esthetic reasons, it is coated witha dental ceramic or acrylic veneering material to provide the form,color and contour of the dentition. The metal structure is frequentlycast into a mold prepared from an inorganic investment material, but itmay be formed by other methods such as computer assisted design andmachining.

Most often cast metal structure is veneered with a dental ceramic orwith acrylics that gives the prostheses the shape and the shade ofnatural teeth. The veneer material must be very opaque in order to coverthe metal structure of the prosthesis which provides an undesirableappearance. Prior prostheses are not entirely bio-compatible ascorrosion of the metal therein causes them to discolor and inflameadjacent gum tissue, which may also recede. Still another disadvantageis corrosion or solution effects of metal causes discoloration of theveneer or adjacent soft tissue.

A disadvantage of the prior art metal supported ceramic structures isthat the metal often appears as a visible dark border at the boundary ofthe prosthesis in contact with the gingiva. The metal substructures arenot generally completely veneered with ceramic or acrylic in this areain order to utilize the greater edge strength of metals, as well as toprotect the gum against damages caused by thick margins, and as aconsequence a metallic colored ring at the margin of the restorationresults in less than optimal esthetic results. Another disadvantage ofprosthesis which have a metal substructure is that, while the highlyopaque thin coating of ceramic or acrylic obscures the influence ofgray-silvery metal used to simulate the tooth color and shape, thisopaque thin layer is often incompletely or inadequately applied so thatthe final restoration appears grayer, greener or otherwise different inshade than the adjacent natural dentition.

Claussen et al in U.S. Pat. Re. 32,449 discloses ceramic body ofzirconium dioxide (ZRO₂) and method for its preparation. Lange in U.S.Pat. No. 4,316,964 discloses Al₂ O₃ /ZRO₂ ceramic. Claussen et al inU.S. Pat. No. 4,525,464 discloses ceramic body of zirconium dioxide(ZrO₂) and method for its preparation. Knapp in U.S. Pat. No. 4,565,792discloses partially stabilized zirconia bodies. Tsukuma et al in U.S.Pat. No. 4,587,225 discloses high-strength zirconia type sintered body.Manniing in U.S. Pat. No. 4,751,207 discloses alumina-zirconia ceramic.Kelly in U.S. Pat. No. 4,978,640 discloses dispersion strengthenedcomposite. Kriechbaum et al in U.S. Pat. No. 5,011,673 discloseszirconium oxide powder. Iwasaki et al in U.S. Pat. No. 5,130,210discloses stabilized zirconia solid electrolyte and process forpreparation thereof. Jacobson in U.S. Pat. No. 5,155,071 disclosesflame-produced partially stabilized zirconia powder. Tyszblat in U.S.Pat. No. 4,772,436 discloses a complicated and time consuming method ofpreparing prosthesis from alumina. Adair and Adair et al in U.S. Pat.Nos. 4,744,757; 4,478,641 and 4,431,420 disclose glass ceramic dentalproducts. Quadir; Masaki et al; David and Otagiai et al in U.S. Pat.Nos. 4,764,491; 4,742,030; 4,520,114 and 4,360,598 respectively disclosezirconia ceramics. Feagin, Guigonis, Sakurai et al and Ivarsson et al inU.S. Pat. Nos. 4,415,673; 4,504,591; 4,506,023; 4,755,228 and 4,806,168respectively disclose refractory materials. Hieke et al in U.S. Pat. No.4,111,711 discloses cements. Ducheyne et al in U.S. Pat. No. 5,120,340discloses bioreactive material for a prosthesis. Adair in CanadianPatent 1,148,306 discloses dental products and processes involving micacompositions. Otagiri et al in Canadian Patent 1,154,793 discloseszirconia ceramic and a method of producing the same. Feagin in CanadianPatent 1,202,333 discloses refractory material. Adair et al in CanadianPatent 1,212,125 discloses embedding material useful in preparingglass-ceramic products. Ivarsson et al in Canadian Patent 1,239,656discloses refractory material and its use. Adair et al in CanadianPatent 1,259,507 discloses fixed partial dentures and method of making.Tyszblat in Canadian Patent Application 1,309,845 discloses procedurefor making a prosthesis. Duchyne et al in Canadian Patent Application2,024,646 discloses material for prosthesis. Grebe et al in CanadianPatent Application 2,072,946 discloses rare earth-containing fritshaving a high glass transition temperature and their use for theproduction of enamels having improved heat resistance. Jones in CanadianPatent Application 2,045,859 discloses compositions. Andrus et al inCanadian Patent Application 2,044,060 discloses coated refractoryarticle and method. Ditz et al in Canadian Patent Application 2,042,349discloses biocompatible glass. Rheinberger et al in Canadian PatentApplication 2,038,695 discloses polymerizable dental materials.Corcilium in Canadian Patent Application 2,037,343 discloses glasscomposition. Kubota et al in Canadian Patent Application 2,033,289discloses alumina-zirconia composite sintered product and method formaking the same. Ricoult et al in Canadian Patent Application 2,031,666discloses transparent glass-ceramic articles. Anderson in CanadianPatent Application 2,010,595 discloses method for producing a ceramicunit. Tsukuma et al in Canadian Patent Application 1,300,178 disclosesceramic orthodontic bracket and process for making same. Akahane et alin Canadian Patent 1,292,017 discloses glass powders for dental glassionomer cements. Manning in Canadian Patent 1,268,490 disclosesalumine-zirconia ceramic. Heurtaux in Canadian Patent 1,258,557discloses basal ceramic layer for opacifying the metal coping of aceramo-methallic dental reconstruction. Howard in Canadian Patent1,234,163 discloses support particles coated with precursors forbiologically active glass. Manning in Canadian Patent 1,232,620discloses alumina ceramic comprising a siliceous binder and at least oneof zirconia and hafnia. Beall et al in Canadian Patent 1,196,032discloses transparent glass ceramic containing mullite. Richez inCanadian Patent 1,195,702 discloses Material bioreactives. Potter et alin Canadian Patent 1,189,092 discloses glasses. Schmitt et al inCanadian Patent 1,156,679 discloses calcium aluminum fluorosilicateglass powder. Starling et al in Canadian Patent 1,146,980 disclosesceramic dental appliance and method and dental ceramic for themanufacture thereof. Perez in Canadian Patent 1,129,688 disclosesinternal ceramic core. Barrett et al in Canadian Patent 1,120,960discloses glass-ceramic dental restorations. Gagin in Canadian Patent1,105,498 discloses alkali-resistant glass fiber composition. Neely inCanadian Patent 1,078,412 discloses low pollution glass fibercompositions. Ohtomo in Canadian Patent 1,074,341 disclosesalkali-resistant glass composition and glass fibers made therefrom. Sungin Canadian Patent 1,053,408 discloses dental bonding agents. Deeg et alin Canadian Patent 1,047,756 discloses faraday rotation glasses. Ohtomoin Canadian Patent 1,040,222 discloses alkali resistant glass. Atkinsonet al in Canadian Patent 1,015,778 discloses glass compositions andfibers made therefrom. Wolf in Canadian Patent 1,013,775 discloses glasscomposition. Hancock et al in Canadian Patent 997,791 discloses sinteredzirconia bodies. Tamamaki et al in Canadian Patent 2,059,402 disclosesfused Alumina-zirconia-yttria refractory materials. Tamamaki et al inCanadian Patent 2,044,041 discloses fused zirconia refractory materialshaving high-temperature heat resistance and corrosion resistance and amethod for producing the same. Morishita in Canadian Patent 1,281,340discloses zirconia ceramics and a process for production thereof. Matsuoet al in Canadian Patent 1,273,648 discloses refractory material andcastable refractory for molten methal container. Bush et al in CanadianPatent 1,272,491 discloses magnesia partially-stabilized zirconia.Colombet et al in Canadian Patent 1,259,079 discloses zirconiastabilizers. Guile in Canadian Patent 1,236,855 discloses stabilizedzirconia bodies of improved toughness. Sugie in Canadian Patent1,228,372 discloses process for producing a zirconia refractory body anda product produced by the process. Knapp in Canadian Patent 1,216,007discloses partially stabilized zirconia bodies. Garvie et al in CanadianPatent 1,135,728 discloses partially stabilized zirconia ceramics.Schulz et al in Canadian Patent 1,134,869 discloses thixotropicrefractory binder based upon aluminum phosphate gelled silica sols.Garvie et al in Canadian Patent 1,053,709 discloses ceramic materials.Linton in Canadian Patent 1,041,557 discloses acid and heat-resistantmortars for cellular glass. Labant et al in Canadian Patent Application2,037,372 discloses enamel compositions. Becker in Canadian PatentApplication 2,017,884 discloses glass composition. Klaus et al inCanadian Patent 1,279,154 discloses dental compositions fired dentalporcelains and processes for making and using same. Bailey et al inCanadian Patent 1,275,185 discloses Bonding glass-ceramic dentalproducts. Klimas et al in Canadian Patent 1,274,857 discloses lead-freeglass frit compositions. Katz in Canadian Patent 1,272,222 discloseshigh strength dental porcelains. Heurtaux in Canadian Patent 1,251,306discloses ceramic intermediate layer for a ceramo-methallic dentalreconstruction. Heurtaux in Canadian Patent 1,251,305 disclosestransparent ceramic surface layer for a ceramo-metallic dentalreconstruction. Francel et al in Canadian Patent 1,232,619 discloseslead-free and cadmium-free glass frit compositions for glazing,enameling and decorating. Francel et al in Canadian Patent 1,212,970discloses lead-free and cadmium-free glass frit composition for glazing,enameling and decorating. Hagy et al in Canadian patent 1,156,684discloses very low expansion sealing frits. Eppler in Canadian Patent1,141,396 discloses low-melting, lead-free ceramic frits. Chaung inCanadian Patent 1,212,302 discloses method for etching dental porcelain.Prall in Canadian Patent 1,138,155 discloses cordieritecrystal-containing glaze. Berneburg in Canadian Patent Application2,020,486 discloses aluminum oxide ceramic having improved mechanicalproperties.

It is known to use dental ceramic as replacements for metal but mostoften the resulting product has been too weak to fulfill the neededmechanical strength in practice, or else the procedure used has beenexcessively difficult and unreliable.

It is an object of the invention to provide a material and method forproducing a completely non-metallic prosthesis comprising a ceramicframe material and an esthetic ceramic veneer material and methods fortheir manufacture.

It is an object of the invention to provide ceramic/glass dentalcompositions which are adapted to be molded at high pressure whileheating to form prosthesis and prosthetic parts.

It is an object of the invention to provide compositions of zirconiaand/or alumina powders, and glass powder which are molded to providehigh strength dental prosthetic components and parts.

It is an object of the invention to provide dental prosthesis andprosthetic parts by molding compositions of zirconia or alumina powdersand their mixtures with an admixture of powdered silicate andaluminosilicate glasses under heat and pressure to produce high strengthprosthetic components and parts.

It is an object of the invention to provide compositions of zirconiaand/or alumina powders, silicate and/or aluminosilicate glass powders,which are molded to produce high strength dental prosthetic componentsand parts veneered with esthetic dental ceramics.

It is an object of the invention to provide ceramic veneering materialssuitable for providing esthetic ceramic coatings to the moldings of thisinvention.

It is the object of the invention to provide a process for formingceramic/glass dental prosthesis and prosthetic components and partshaving superior esthetic and strength properties.

It is an object of the invention to provide a process for molding dentalprosthesis and prosthetic components and parts from a moldingcomposition which includes ceramic and glass, powders while heatingunder pressure.

It is an object of the invention to provide a process for moldingzirconia and/or alumina powders and glass powder to provide highstrength prosthetic components and parts.

It is an object of the invention to provide a process for moldingzirconia and/or alumina powders and silicate and/or aluminosilicateglass powder to produce high strength prosthetic components and partswhich are then coated with a slurry of dental ceramic, and fired.

It is the object of the invention to provide investment materialssuitable for preparing molds within which the ceramic/glass compositionsof the invention are molded.

It is an object of the invention to provide molding apparatus suitablefor molding dental prosthesis from the ceramic/glass materials of theinvention at elevated temperatures and high pressure.

It is an object of the invention to provide dental equipment suitablefor molding dental prosthesis from the ceramic/glass materials of theinvention at elevated temperatures and high pressure while vacuum isapplied to the mold.

Mbar as used herein means millibar.

Dental prosthesis as used herein means any article of manufacture usedto replace a missing element of the oral cavity, especially the hard andadjacent soft tissues, teeth and gingiva including caps, crowns,veneers, bridges, inlays, onlays, and dentures or any part thereof.

Throughout this disclosure all percentages are percent by weight unlessotherwise indicated.

BRIEF DESCRIPTION OF THE INVENTION

A shaped, high-strength dental ceramic prosthesis is provided by moldinga composition which includes from about 1-50 percent by weight glassparticles and from 50-99 percent by weight of ceramic particles atpressures up to about 40 MPa and temperatures up to about 1200° C.

BRIEF DESCRIPTION OF THE FIGURES

FIG. 1 is a partial side view of a press for use in making prostheses inaccordance with the invention.

FIG. 2 is a side view of a press for use in making prostheses inaccordance with the invention.

FIG. 3 is a partial top view along line A--A in FIG. 2 of a press foruse in making prostheses in accordance with the invention.

FIG. 4 is a partial cross-sectional top view along line B--B in FIG. 1of a press for use in making prostheses in accordance with theinvention.

DETAILED DESCRIPTION OF THE INVENTION

The invention provides a prosthesis by molding ceramic/glass powdercompositions under heat and pressure. A prosthesis is made by forming awax or wax-substitute into a model of the shape and size of the ceramicprosthesis to be formed. This model is then surrounded with aninvestment material within a mold ring. The investment material isallowed to harden while being thermally conditioned. The model isphysically removed or burned out of the hardened investment material athigh temperature to leave a mold with a cavity having the shape and sizeof the dental prosthesis (molding) to be formed. The ceramic/glassmolding powder composition is then transferred to the cavity and heatedunder pressure to produce a molded dental prosthesis.

In an embodiment of the invention vacuum is applied to the mold duringmolding and/or heating. The time and rate of cooling of the moldeddental prosthesis in the investment material is controlled so that themolding is tempered to relieve internal stresses. The molded dentalprosthesis is subsequently divested. The dental prosthesis preferably isveneered with an additional lower-temperature-forming ceramic to producean individually characterized prosthesis with natural appearingvariations in color and translucency to most aesthetically matchadjacent teeth. Veneering ceramics are used which have good adhesion tothe molded dental prosthesis, and approximately the same coefficient ofthermal expansion. The veneering compositions are applied directly tothe molded dental prosthesis as a slurry of powder and aqueous liquid,which is then fired at a temperature lower than the molding temperatureof the molded dental prosthesis.

CERAMIC/GLASS MOLDING COMPOSITION

Thus, in accordance with the invention high strength dental prosthesesare formed, which do not require metal substructure, by applyingpressure and heat to a ceramic/glass powder composition in a mold. Theceramic/glass powder composition preferably includes alumina powderand/or zirconia powder, and a glass powder. It has been found that theaddition of the glass facilitates molding highly refractory ceramicpowders such as alumina and zirconia at sintering temperatures lowerthan the ceramic powders alone while surprisingly providing higherstrength prosthesis. Preferably the ceramic powder has a melting pointwhich is at least 1000° C. higher than the softening temperature of theglass powder. Preferably the ceramic/glass molding powder compositionincludes from about 50 to about 99 parts by weight alumina and/orzirconia powder and from about 1 to about 50 parts by weight of glasspowder. Based upon the chemical nature of the glass, and its physicalproperties including softening point and viscosity, the ceramic/glassmolding composition is fired at a temperature between 800 and 1300° C.To assure condensation of the ceramic/glass molding composition,pressures of up to about 40 MPa, are applied during firing. The pressureis applied for optimal condensation of the ceramic/glass composition.Preferably a vacuum is applied to the mold before and/or during firingto remove occluded air from and through the investment material,ceramic/glass molding composition and the mold cavity to assist informing prostheses with reduced porosity and greater strength. Theflexural strengths of the prostheses produced in accordance with theinvention are at least greater than about 150 MPa, and more preferablygreater than 300 MPa; most preferably greater than 500 MPa.

In a preferred embodiment a powdered ceramic/glass molding compositionof the invention includes from about 50 to 99 parts by weight ofpowdered alumina and/or yttrium stabilized zirconia and 1 to 50 parts byweight of aluminosilicate glass. Preferably, components of ceramic/glassmolding compositions of the invention are sufficiently mixed tosubstantially evenly distribute the glass and ceramic particles. Morepreferably the ceramic/glass molding composition includes from 60 to 85parts by weight of powdered alumina or yttrium stabilized zirconia and15 to 40 parts by weight of aluminosilicate glass powder. Mostpreferably the ceramic/glass molding composition includes from 65 to 80parts by weight of powdered alumina or yttrium stabilized zirconia withparticles sizes less than 35 microns and 20 to 35 parts by weight ofaluminosilicate glass.

In another preferred embodiment of a ceramic/glass molding compositionof the invention includes from about 10 to 70 parts by weight (pbw) ofpowdered yttrium stabilized zirconia, 10 to 70 parts by weight aluminaand 1 to 50 parts by weight of lanthanum borosilicate glass powder. Morepreferably a ceramic/glass molding composition of the invention includesfrom 20 to 50 pbw of powdered yttrium stabilized zirconia, 20 to 50 pbwalumina, and 15 to 40 parts by weight of lanthanum aluminoborosilicateglass powder. Most preferably a ceramic/glass molding composition of theinvention includes from 20 to 50 pbw of powdered yttrium stabilizedzirconia with particles sizes less than 35 microns, 20 to 50 pbw aluminawith particles less than 35 μm, and 20 to 35 parts by weight oflanthanum aluminoborosilicate glass, principally in powder form withparticles less than 100 μm.

In another preferred embodiment a molding powder is prepared from 50 to99 parts by weight of powdered alumina and/or yttrium stabilizedzirconia which are overcoated with from 1 to 50 parts by weight of asilicate or aluminosilicate glass which in a preferred embodiment is alanthanum borosilicate glass and comminuted to powder to which is thanadmixed 1 to 50 parts by weight of a second powdered silicate oraluminosilicate glass. In a more preferred embodiment 50 to 90 pbwalumina or yttrium stabilized zirconia are overcoated with 1 to 40 pbwlanthanum borosilicate glass and comminuted to powder to which isadmixed 10 to 50 pbw of a powdered aluminosilicate glass. In a mostpreferred embodiment 50 to 85 pbw alumina or yttrium stabilized zirconiaare overcoated with 1 to 30 pbw lanthanum borosilicate glass andcomminuted to powder to which is admixed 15 to 35 pbw of a powderedaluminosilicate glass.

Preferably the average particle size of the ceramic powder of theceramic/glass molding composition is less than 35 μm, more preferablyless than 10 μm and most preferably less than 5 μm. These particlespreferably have a distribution of sizes that leads to close packing.

The glass powder particles of the ceramic/glass molding compositionpreferably have a coefficient of thermal expansion between 3 and 15×10⁻⁶per K. at temperatures between 25 and 400° C., and an average particlesize less than 100 μm, more preferably less than 35 μm and mostpreferably less than 5 μm. Preferably the glass of the ceramic/glassmolding composition includes lanthanum, aluminum, boron, silicon,calcium, zirconia, yttrium and small amounts of other elements whichreadily form cations.

In a preferred embodiment of the invention a lanthanum borosilicateglass component of the glass/ceramic molding composition includes:

5 to 30 percent by weight of SiO₂,

5 to 25 percent by weight of B₂ O₃,

5 to 30 percent by weight of Al₂ O₃,

20 to 60 percent by weight of La₂ O₃,

0 to 15 percent by weight of CaO,

0 to 15 percent by weight of ZrO₂, and

0 to 15 percent by weight of Y₂ O₃.

In a more preferred embodiment of the invention the lanthanumborosilicate glass of the glass/ceramic molding composition includes:

10 to 25 percent by weight of SiO₂,

10 to 20 percent by weight of B₂ O₃,

10 to 20 percent by weight of Al₂ O₃,

30 to 50 percent by weight of La₂ O₃,

0 to 10 percent by weight of CaO,

0 to 10 percent by weight of ZrO₂, and

0 to 10 percent by weight of Y₂ O₃.

In a most preferred embodiment of the invention the lanthanumborosilicate glass of the glass/ceramic molding composition includes:

15 to 22 percent by weight of SiO₂,

12 to 18 percent by weight of B₂ O₃,

14 to 20 percent by weight of Al₂ O₃,

35 to 45 percent by weight of La₂ O₃,

0 to 5 percent by weight of CaO,

0 to 5 percent by weight of ZrO₂, and

0 to 5 percent by weight of Y₂ O₃.

Preferably a glass is of low viscosity above its softening point and isof suitable composition to optimally wet the alumina or zirconiacomponents of the ceramic/glass molding compositions. It is believed theglass acts as a plasticizer and lubricant allowing the alumina and/orzirconia powders to surprisingly allow the composition to besubstantially compressed at relatively low temperatures of less thanabout 1200° C. and pressures less than about 40 MPa, and thus to bemolded to form high strength prosthesis of irregular shape.

In another embodiment of the invention, whereby glass and ceramicpowders are mixed and molded directly at the herein before mentionedtemperature and pressures it has been found that high strength ceramicparticles of zirconia and alumina may be first advantageouslyover-coated with a minor portion of a glass of the type herein beforedescribed capable of wetting the ceramic particles by fusing with theglass and then comminuting to form a powder. The powder so formed isthen mixed with a second portion of particulate glass powder of the samecomposition, or alternatively another silicate or aluminosilicate glasssoluble with the first, by which means the molding of the composition isfacilitated. In an embodiment thereof a second glass is analuminosilicate glass which in a preferred embodiment has the followingcompositional ranges:

    ______________________________________                Percent by                weight    ______________________________________            SiO.sub.2                  65-69            Al.sub.2 O.sub.3                   9-12            K.sub.2 O                   7-10            Na.sub.2 O                  6-9            LiO.sub.2                  1-2            CaO   2-4            BaO   0-1            F     0-1            CeO.sub.2                   0-.5    ______________________________________

After molding, the glass phase of the molded prosthesis is preferablyconditioned with acids or alkali to obtain a microretentive etchingpattern so that adhesive bonding to a tooth with a luting cement orcomposite is achieved. Likewise the ceramic/glass molding is preferablytreated with well known silanes, for example,3-methacryloyloxpropyltrimethoxy silane to provide an interactivesurface with luting composites to assist in bonding the prosthesis tothe luting cement or luting composite.

INVESTMENT MATERIAL COMPOSITIONS

According to the invention investment materials are provided which areof suitable high strength and physical characteristics to withstand therequired molding pressures and temperatures and have coefficients ofthermal expansion corresponding to that of the molded composition toallow the hot molded prosthesis to cool to a high strength articlewithout "freezing in" excessive differential stresses that might crackor spall the article. Accordingly, in a preferred embodiment of theinvention the coefficient of thermal expansion of the investmentmaterial is between 3×10⁻⁶ and 15×10⁻⁶ per K more preferably between4×10⁻⁶ and 13×10⁻⁶ per K, and most preferably 5×10⁻⁶ and 12×10⁻⁶ per Kbetween 25 and 1100° C.

The compressive strength of the investment material must be greater thanthe applied force upon the molding material applied by the apparatus atthe molding temperature. Preferably the compressive strength is greaterthan about at least 15 MPa. Higher strengths at the molding temperaturewill allow greater pressures to be used in molding the ceramic/glasscomposition. For convenience in molding and for greatest accuracy thechange in dimension in the investment upon hardening around the waxmodel, whether due to thermal effects or chemical changes, is preferablyless than 2%.

In one embodiment of the invention investment material compositions areprepared from a powder and a liquid which are mixed at the time the moldis to be prepared, under vacuum, for about a minute until a homogeneousmass is obtained. Preferably 100 g powder are mixed with from 5 to about50 parts by weight of liquid, more preferably 10 to 30 ml liquid, andmost preferably 15 to 20 ml liquid. The liquid includes aqueous silicasol preferably having from 5 to 60 percent by weight silica. Morepreferably the aqueous silica sol has from 20 to 50 percent by weightsilica. Most preferably the aqueous silica sol has from 25 to 45 percentby weight silica. The powder preferably includes a filler, eithercalcium or magnesium stabilized zirconia, magnesium oxide, quartz,cristobalite, fused silica, alumina and calcium fluoride or blendsthereof. Optionally, the powder includes a binder for the filler whichincludes magnesia and mono- or diammonium hydrogen phosphate ormagnesium hydrogen phosphate or blends thereof. Fillers are selected toobtain high compressive strength and coefficient of thermal expansionsubstantially equal to that of the ceramic/glass molding composition. Inone embodiment of the invention calcium stabilized zirconia is preferredbecause a partial phase transformation at high temperatures reduces theshrinkage of the investment material.

A binder is preferably added to adjust the coefficient of thermalexpansion and increase green strength. Preferably such binder has aweight ratio of magnesia to phosphates of from 0.5:1.5 to 1.5:0.5, andmore preferably about 1:2. Preferably from 0 to 15 percent by weight ofglass/ceramic powder composition is binder, and more preferably thebinder is from 1 to 10 percent by weight.

PREPARATION OF THE MOLD

A metal mold ring, of size appropriate for the part to be molded, islined with a layer of refractory felt saturated with water and is placedon a work surface. The model wax-up of the part to be molded is placedwithin the ring and partially embedded in newly mixed investmentmaterial. A layer of petroleum jelly, nail polish or other separator isapplied to the surface of the mold when the investment is sufficientlyhard that it can support the weight of a second mix poured over thesurface. Hardening may require up to about 2 hours depending on thecomposition. A second mix is then poured against the surface of thefirst. Heat is then applied to the mold to remove water. At first theheat is applied slowly and carefully to allow the water to evaporatewithout causing cracks in the investment. Later more heat is applied sothat the wax-up and other organic materials burn completely. Theinvestment temperature is raised again, to about 1000 to 1300° C. for30-90 minutes to strengthen the investment. A controlled cool-down cyclemay be necessary to prevent cracking. Thereafter the mold is opened andthe parting surfaces of the mold are coated with a refractory separatingagent. In one preferred embodiment a thin layer of graphite is appliedfrom a organic liquid (acetone) dispersion thereof as a separatingagent.

FILLING THE MOLD

The amount of ceramic/glass powder molding composition required to formthe molded dental prosthesis is determined by weighing the model wax-up.The weight of ceramic/glass powder molding composition required to fillthe mold corresponds to the weight of the wax-up material. A conversionchart or table is preferably prepared by converting weight of wax-upmaterial to the corresponding number of grams of molding powdercomposition, based upon the specific gravities of both materials. Themold cavity is charged with the amount of ceramic/glass moldingcomposition corresponding to the amount of wax, (the chart isconveniently used for this determination) required for the moldedprosthesis, and preferably a predetermined excess of ceramic/glassmolding composition is added to the mold cavity.

The lower half of the mold is charged with ceramic/glass molding powdercomposition or optionally a water based slurry thereof and the halvesbrought together, and then is placed into the molding apparatus upon theplatten of the pressure ram. In one preferred embodiment the moldingcomposition is compressed while being heated, under vacuum applied tothe mold, for the requisite time and temperature to form the moldedprosthesis or prosthetic part.

MOLDING

In accordance with the invention molding of the ceramic/glass powdercomposition is carried out in a device that can apply the necessarymolding forces at high temperature. Pressures of up to about 40 MPa arepreferred since the ceramic/glass composition can be highly viscous evenat elevated molding temperatures. In one embodiment of the invention ofthe ceramic/glass powder composition a vacuum is optionally appliedwithin the mold and mold cavity to facilitate packing, remove watervapor, and to reduce air that might otherwise mix with the molten glasscomposition and cause increased porosity in the finished molding.Preferably, this vacuum applied within the mold cavity is up to about 20mbar and more preferably up to about 40 mbar.

MOLDING APPARATUS

FIGS. 1-4 show a press system 10 in accordance with the invention havingan optional press ring 12 circumscribing mold 14. Mold 14 has upper andlower mold halves 16 and 18 respectively each made of investmentmaterial and positioned within press ring 12. Above mold 14 is counterpressure body 20 which is made either of a ceramic, for example aluminumoxide or high strength metal. The mold 14 is supported on top of theround table 22 inside a vacuum-firing chamber exterior wall 24. Roundtable 22 is adapted to be lifted by a rod 26 toward counter pressurebody 20 and press against press top 28.

With more particular reference to FIG. 2, it is seen that press system10 has movable cross-head 30, columns 32, and hydraulic pressure unit40. Hydraulic unit 40 provides force on mold 14 through rod 26. Theforce from hydraulic unit 40 is transmitted through rod 26 onto table22. The mold 14 contains ceramic/glass powder and is positioned withinchamber exterior wall 24. Mold 14, has a mold cavity within wall 42.Removable counter-support 20 includes a high strength metal member 46and end plattens 47, and is circumscribed by a low thermal diffusivitymember 48 which is made of insulating material, such as aluminum oxidefiber or a light weight silica refractory brick. In FIG. 1 the metalmembers are rods passing through the refractory brick.

Mold 14 is enclosed by vacuum-firing chamber lid 50 and vacuum chamberwall 24 which when in contact with flange 56 form a seal so that avacuum may be drawn within the chamber formed thereby. Heating elements52 are positioned adjacent to mold 14, and within insulating material54. Chamber wall 24 is supported by flange 56 fitted with a gasket 58.Flange 56 is connected to hydraulic unit 40. Pressure ram 60 isconnected to hydraulic pressure unit 40 by means of rod 26 and extendsthrough flange 56.

Crosshead 30 is adapted to rotate as shown in FIG. 3, to allow removalof the vacuum-firing chamber lid 50. Pressure to close the mold issupplied through a pedestal 62 which is removably connected to pressureram 60. Pedestal 62 includes a high strength member 64 surrounded byinsulation 66. The pressure required to close and compress the contentsof the mold is resisted by a counter-support 80 which includes a highstrength metal member 81 surrounded by insulation 82. Vacuum is drawn byvacuum pump 84 through the vacuum-firing chamber lid 50. After moldingcrosshead 30 is rotated, vacuum-firing chamber lid 50 and countersupport 68 are removed to permit access to the mold 14.

Alternative configurations are possible and will be obvious to thoseskilled in the art. For example, and not limited hereto, the motivepowder to activate the pressure ram may be derived using pneumatic,hydraulic or mechanical elements such as a pneumatic cylinder, hydrauliccylinder, or threaded spindle. Likewise, the molding operation mightcomprise transfer or injection molding techniques alone or incombination with the compression molding technique described above.

VENEERING THE CERAMIC/GLASS PROSTHESIS

Although the prosthesis and prosthetic parts formed using thecompositions of this invention are not silvery-gray and thus arethemselves superior in esthetics to metal substructures, their estheticcharacter may be desirably enhanced in certain cases, as for example,the replacement of an anterior tooth, where the apparent color mustclosely approximate that of the adjacent teeth. Thus powdered ceramiccompositions of superior translucency are colored to approximate naturaltooth colors, and applied to a molded dental prosthesis to veneer itssurface. Such veneer compositions must wet the molded dental prosthesisand have coefficients of expansion substantially equal to thecoefficient of thermal expansion of the prosthesis to minimize internalstresses.

According to one embodiment of the invention the ceramic/glass part isveneered using shaded dental feldspathic ceramic powder compositions orother ceramic compositions slurried with water or a water solution ofsalts or polymers and painted or otherwise applied to the part to beveneered by methods known in the art, and fired in an oven attemperatures less than the molding temperatures of the ceramic/glasspart. A high strength laminar composite is formed which is withoutcracks and checks and is resistant to breakage. To achieve these resultsceramic materials have coefficients of thermal expansion similar to thesubstrate of ceramic/glass material. Thus in a preferred embodiment theveneering composition has coefficients of thermal expansion between3×10⁻⁶ and 13×10⁻⁶ per K, more preferably between 4×10⁻⁶ and 12×10⁻⁶ perK and most preferably between 6×10⁻⁶ and 10×10⁻⁶ per K. Averageparticles sizes are preferable within the range of 1-40 μm, morepreferable 5-35 μm, and most preferable 10-30 microns.

Ceramic powders suitable for use as veneering materials may be preparedfrom a single frit or combinations of two or more frits. In a preferredembodiment thereof, ceramic frits of two different coefficients ofexpansion are provided, such that the powdered frits can be combined invarious proportions to match the coefficient of thermal expansion of theceramic/glass part, also known as the substrate.

In a preferred embodiment of the invention at least two frits arecombined to form a ceramic. One such frit (Frit I) preferably includes:

65 to 75 percent by weight of SiO₂,

10 to 16 percent by weight of Al₂ O₃,

7 to 11 percent by weight of K₂ O,

2 to 5 percent by weight of Na₂ O,

0 to 2 percent by weight of Li₂ O,

1 to 4 percent by weight CaO,

0 to 2 percent by weight of B₂ O₃,

0.5 to 1 percent by weight of Tb₂ O₃, and

0.1 to 0.3 percent by weight of CeO₂.

More preferably such frit includes:

67 to 71 percent by weight of SiO₂,

12 to 15 percent by weight of Al₂ O₃,

8 to 10 percent by weight of K₂ O,

3 to 5 percent by weight of Na₂ O,

0 to 1 percent by weight of Li₂ O,

1 to 3 percent by weight of CaO,

0 to 1 percent by weight of B₂ O₃,

0.5 to 1 percent by weight of Tb₂ O₃, and

0.1 to 0.3 percent by weight of CeO₂.

Another such frit (Frit II) preferably includes:

67 to 77 percent by weight of SiO₂,

8 to 12 percent by weight of Al₂ O₃,

6 to 10 percent by weight of K₂ O,

2 to 5 percent by weight of Na₂ O,

0 to 2 percent by weight of Li₂ O,

1 to 4 percent by weight CaO, and

1 to 4 percent by weight of B₂ O₃.

More preferably such frit includes:

70 to 75 percent by weight of SiO₂,

9 to 11 percent by weight of Al₂ O₃,

8 to 10 percent by weight of K₂ O,

2 to 4 percent by weight of Na₂ O,

0 to 1 percent by weight of Li₂ O,

1 to 3 percent by weight of CaO, and

2 to 3 percent by weight of B₂ O₃.

These ranges of composition of veneering ceramics frits I and II arecombined to form a ceramic of suitable properties for veneering theceramic/glass substrate. The coefficient of thermal expansion of frit Iis about 8×10⁻⁶ per K and the coefficient of thermal expansion of fritII is about 6.7×10⁻⁶ per K.

Veneering ceramics are fired in conventional furnaces by conventionalmeans using for example a Multimat MCII furnace (Dentsply Gmbh) in 2-5minutes under vacuum. Firing ranges are 900-1100° C., preferably950-1050° C., most preferably 980-1020° C. The ceramics may be shadedwith pigments used in the ceramic industry for example SnO₂, ZrSiO₄,Zn--Fe--Cr--Al (Spinel), and Zr--Si--Pr (Zircon). These veneeringceramics have flexural strengths greater than 50 MPa and solubility lessthan 0.05, values which comply with ISO 6872.

The invention is illustrated by several examples. It should beunderstood that these examples are not intended to limit the scope ofthe invention, but to be illustrative only.

EXAMPLE 1 PREPARATION OF INVESTMENT MATERIAL

An investment material is formulated from a powder and a liquid, andused to prepare the molds in which the ceramic/glass moldingcompositions of Examples 3 through 7, 9 and 10 are molded. The powdercomponent is comprised of 1.0 gram magnesium oxide, 1.0 gram ammoniummonophosphate and 98.0 grams of calcium stabilized zirconium oxide (ZrO₂--CaO 95/5, Lonza Gmbh, Germany) which has particle sized less than 100μm and has the following chemical composition:

    ______________________________________                Percent by                Weight    ______________________________________            ZrO.sub.2                  93.8            CaO    5.0            MgO   <0.1            SiO.sub.2                  <0.4            Al.sub.2 O.sub.3                  <0.4            Fe.sub.2 O.sub.3                  <0.1            TiO.sub.2                  <0.2    ______________________________________

The liquid component is an aqueous silica sol (Lewasil 200, 30% BayerAG, Germany). The sol is 30 percent by weight of silica. A model of wax(wax-up) of the dental prosthesis to be molded is prepared. A mold ringis placed on a work surface. A 1 mm thick lining of refractory felt(Kaoliner, Dentsply Gmbh) saturated with water is placed within thering. 100 g powder and 17 ml liquid investment material are mixed undervacuum for about 1 minute until a homogeneous mass is obtained. Aportion of newly mixed investment material, sufficient for embeddinghalf the model (wax-up) of the object to be molded is poured into themold and allowed to stand until it hardens. This requires approximately60 minutes. The surface of the hardened investment material is coatedwith a petroleum jelly as a separating medium upon which newly mixedinvestment material is poured to fill the mold ring and allowed toharden. After 5 hours the mold is heated in an oven as follows: 300° C.at a rate of 4° C. per minute, then maintained at 300° C. for 0.5 hours,then heated to 1100° C. at a rate of 9° C. per minute, and thenmaintained at that temperature for 1 hour. The process removes water,burns out the model wax, controls shrinkage to 1% and strengthens theinvestment material into a mold. The investment material of the mold hasa thermal expansion of 8.8×10⁻⁶ per ° C. and shrinkage upon sintering ofless than 1%, and a compressive strength of greater than 25 MPa.

EXAMPLE 2 MOLDING PROCEDURE

The mold formed in Example 1 is allowed to cool to room temperature andopened to reveal the mold cavity. A separating agent in the form of aslurry of graphite powder (Leit-C nach Goecke, Neubauer Chemikalien) isapplied to the parting surfaces and cavity of the mold.

The quantity of ceramic/glass molding powder composition required tofill the mold is determined from the weight of the wax up, and placedinto the lower half of the prepared mold. The upper mold half is placedon top of the lower half of the mold, and both halves are placed withinthe vacuum chamber of the press system described herein above and shownin FIGS. 1-4. The counter pressure support, the firing chamber lid, andthen the rotatable crossmember are put in position to lock the assembly.40(+/-20) mbar vacuum is applied, and the mold heated to a temperatureof 1100(+/-50)° C. at a rate of 15-20° C. per minute at whichtemperature a pressure of 25 MPa gauge pressure is applied. Thistemperature and pressure are maintained for from 25 to 30 minutes. Themold and the molded dental prosthesis therein are cooled to 800° C. at arate of 10-15° C. per minute, after which they are cooled to 600° C. ata rate of 1° C. per minute during which time the composition of thedental prosthesis is tempered to optimize strength. Following this themold is cooled to 200° C. at a rate of 10-15° C. per minute, whereuponthe press system is opened to remove the mold and divest the dentalprosthesis. Any investment material clinging to the dental prosthesis isremoved by sandblasting.

EXAMPLE 3 PREPARATION OF DENTAL PROSTHESIS FROM A COMPOSITION OFZIRCONIA AND GLASS

90 g of zirconium dioxide powder is mixed with 10 g of lanthanumboroaluminosilicate glass powder to form a ceramic/glass molding powdercomposition. The zirconium dioxide powder used to make the ceramic/glassmolding powder composition is tetragonal zirconia polycrystalsstabilized with 5% by weight of yttrium oxide (Tosoh Corporation Tokyo,Japan) having a crystal size of 26 nanometer (nm) and a coefficient ofthermal expansion of 10×10⁻⁶ per K and an average particle size of from0.1 to 5 μm. The glass powder used to make the ceramic/glass moldingpowder composition is a lanthanum aluminoborosilicate glass having acoefficient of thermal expansion of 6.2×10⁻⁶ per K and an averageparticle size less than 5 μm, and has the following chemicalcomposition:

    ______________________________________                Percent by                weight    ______________________________________            SiO.sub.2                  18.4            B.sub.2 O.sub.3                  14.3            Al.sub.2 O.sub.3                  16.4            La.sub.2 O.sub.3                  40.9            CaO    2.8            ZrO.sub.2                   4.1            Y.sub.2 O.sub.3                   3.1    ______________________________________

This ceramic/glass composition is put in a mold (made of investmentmaterial as described in Example 1). The mold with the ceramic/glasscomposition therein is positioned within a press system as described inExample 2 and then heated to 1100° C. This temperature is maintained for30 minutes while a pressure of 25 MPa and a vacuum of 40 mbar areapplied to the mold to form a molded dental prosthesis. The mold andmolded dental prosthesis therein are then cooled to 800° C. at a rate of10-15° C. per minute. Then they are cooled at a tempering rate of 1° C.per minute from 800 to 600° C. which avoids the creation of tension atthe glass transition temperature. Following this tempering they arecooled from 600° to 200° C. at a rate of 10-15° C. per minute. They arethen removed from the press system and the mold is divested from thedental prosthesis. The dental prosthesis has a flexural strength of 470MPa when tested according to ISO 6872.

EXAMPLE 4 PREPARATION OF DENTAL PROSTHESIS FROM A COMPOSITION OFZIRCONIA AND GLASS

80 g of the zirconium dioxide powder (used in Example 3) is mixed with20 g of the lanthanum boroaluminosilicate glass powder (used in Example3) to form a ceramic/glass composition. This ceramic/glass compositionis put in a mold (made of investment material as described in Example1). The mold with the ceramic/glass composition therein is positionedwithin a press system as described in Example 2 and then heated to 1100°C. This temperature is maintained for 30 minutes while a pressure of 25MPa and a vacuum of 40 mbar are applied to the mold to form a moldeddental prosthesis. The mold and molded dental prosthesis therein arethen cooled to 800° C. at a rate of 10-15° C. per minute. Then they arecooled at a tempering rate of 1° C. per minute from 800 to 600° C. whichavoids the creation of tension at the glass transition temperature.Following this tempering they are cooled from, 600° to 200° C. at a rateof 10-15° C. per minute. They are then removed from the press system andthe mold is divested from the dental prosthesis. The dental prosthesishas a flexural strength of 550 MPa when tested according to ISO 6872.

EXAMPLE 5 PREPARATION OF DENTAL PROSTHESIS FROM A COMPOSITION OFZIRCONIA AND GLASS

75 g of the zirconium dioxide powder (used in Example 3) is mixed with25 g of the lanthanum boroaluminosilicate glass powder (used in Example3) to form a ceramic/glass composition. This ceramic/glass compositionis put in a mold (made of investment material as described in Example1). The mold with the ceramic/glass composition therein is positionedwithin a press system as described in Example 2 and then heated to 1100°C. This temperature is maintained for 30 minutes while a pressure of 25MPa and a vacuum of 40 mbar are applied to the mold to form a moldeddental prosthesis. The mold and molded dental prosthesis therein arethen cooled to 800° C. at a rate of 10-15° C. per minute. Then they arecooled at a tempering rate of 1° C. per minute from 800 to 600° C. whichavoids the creation of tension at the glass transition temperature.Following this tempering they are cooled from, 600° to 200° C. at a rateof 10-15° C./per minute. They are then removed from the press system andthe mold is divested from the dental prosthesis. The dental prosthesishas a flexural strength of 620 MPa when tested according to ISO 6872.

EXAMPLE 6 PREPARATION OF DENTAL PROSTHESIS FROM A COMPOSITION OFZIRCONIA AND GLASS

70 g of the zirconium dioxide powder (used in Example 3) is intimatelymixed with 30 g of the lanthanum boroaluminosilicate glass powder (usedin Example 3) to form a ceramic/glass composition. This ceramic/glasscomposition is put in a mold (made of investment material as describedin Example 1). The mold with the ceramic/glass composition therein ispositioned within a press system as described in Example 2 and thenheated to 1100° C. This temperature is maintained for 30 minutes while apressure of 25 MPa and a vacuum of 40 mbar are applied to the mold toform a molded dental prosthesis. The mold and molded dental prosthesistherein are then cooled to 800° C. at a rate of 10-15° C. per minute.Then they are cooled at a tempering rate of 1° C. per minute from 800 to600° C. which avoids the creation of tension at the glass transitiontemperature. Following this tempering they are cooled from, 600° to 200°C. at a rate of 10-15° C. per minute. They are then removed from thepress system and the mold is divested from the dental prosthesis. Thedental prosthesis has a flexural strength of 600 MPa when testedaccording to ISO 6872.

EXAMPLE 7 PREPARATION OF DENTAL PROSTHESIS FROM A COMPOSITION OFZIRCONIA AND GLASS

60 g of the zirconium dioxide powder (used in Example 3) is intimatelymixed with 40 g of the lanthanum boroaluminosilicate glass powder (usedin Example 3) to form a ceramic/glass composition. This ceramic/glasscomposition is put in a mold (made of investment material as describedin Example 1). The mold with the ceramic/glass composition therein ispositioned within a press system as described in Example 2 and thenheated to 1100° C. This temperature is maintained for 30 minutes while apressure of 25 MPa and a vacuum of 40 mbar are applied to the mold toform a molded dental prosthesis. The mold and molded dental prosthesistherein are then cooled to 800° C. at a rate of 10-15° C. per minute.Then they are cooled at a tempering rate of 1° C. per minute from 800 to600° C. which avoids the creation of tension at the glass transitiontemperature. Following this tempering they are cooled from, 600° to 200°C. at a rate of 10-15° C./per minute. They are then removed from thepress system and the mold is divested from the dental prosthesis. Thedental prosthesis has a flexural strength of 300 MPa when testedaccording to ISO 6872. This composition, containing 40% glass, moldedeasily although its strength was somewhat less than 3, 4, 5 and 6. Thus,it is believed that an optimum formulation exists for each formulationof ceramic/glass molding powder.

EXAMPLE 8 PREPARATION OF DENTAL PROSTHESIS FROM A COMPOSITION OF ALUMINAAND GLASS

70 g of alumina powder (Alcoa Chemical and Minerals Inc. Type A-16 whichis 99.7% pure alpha alumina with a surface area of 9 square meters pergram and a crystal size of 0.03-3.5 μm) is mixed with 30 g of lanthanumboroaluminosilicate glass powder of Example 2 to form a ceramic/glasscomposition. This ceramic/glass composition is put in a mold (made ofinvestment material as described in Example 11). The mold with theceramic/glass composition therein is positioned within a press system asdescribed in Example 2 and then heated to 1100° C. No vacuum is drawnupon the mold. This temperature is maintained for 30 minutes while apressure of 25 MPa is applied to the mold to form a molded dentalprosthesis. The mold and molded dental prosthesis therein are thencooled to 800° C. at a rate of 10-15° C. per minute. Then they arecooled at a tempering rate of 1° C. per minute from 800 to 600° C.(which avoids the creation of tension at the glass transitiontemperature). Following this tempering they are cooled from, 600° to200° C. at a rate of 10-15° C./per minute. They are then removed fromthe press system and the mold is divested from the dental prosthesis.The dental prosthesis has a flexural strength of 300 MPa when testedaccording to ISO 6872.

EXAMPLE 9 PREPARATION OF DENTAL PROSTHESIS FROM A COMPOSITION OFZIRCONIA, ALUMINA AND GLASS

56 g of the zirconium dioxide powder (used in Example 3) is mixed with14 g alumina powder (Alcoa Chemical and Minerals Inc. Type A-16 which is99.7% pure alpha alumina with a surface area of 9 square meters per gramand a crystal size of 0.03-3.5 μm) and 30 g of the lanthanumboroaluminosilicate glass powder (used in Example 3) to form aceramic/glass composition. This ceramic/glass composition is put in amold (made of investment material as described in Example 1). The moldwith the ceramic/glass composition therein is positioned within a presssystem as described in Example 2 and then heated to 1100° C. Thistemperature is maintained for 30 minutes while a pressure of 25 MPa isapplied to the mold to form a molded dental prosthesis. The mold andmolded dental prosthesis therein are then cooled to 800° C. at a rate of10-15° C. per minute. Then they are cooled at a tempering rate of 1° C.per minute from 800 to 600° C. (which avoids the creation of tension atthe glass transition temperature). Following this tempering they arecooled from, 600° to 200° C. at a rate of 10-15° C./per minute. They arethen removed from the press system and the mold is divested from thedental prosthesis. The dental prosthesis has a flexural strength of 600MPa when tested according to ISO 6872.

EXAMPLE 10 PREPARATION OF DENTAL PROSTHESIS FROM A COMPOSITION OFZIRCONIA, ALUMINA AND GLASS

42 g of the zirconium dioxide powder (used in Example 3) is mixed with28 g of alumina powder (Alcoa Chemical and Minerals Inc. Type A-16 whichis 99.7% pure alpha alumina with a surface area of 9 square meters pergram and a crystal size of 0.03-3.5 μm) 30 g of lanthanumboroaluminosilicate glass powder of Example 2 to form a ceramic/glasscomposition. This ceramic/glass composition is put in a mold (made ofinvestment material as described in Example 11). The mold with theceramic/glass composition therein is positioned within a press system asdescribed in Example 2 and then heated to 1100° C. This temperature ismaintained for 30 minutes while a pressure of 25 MPa and a vacuum of 40mbar are applied to the mold to form a molded dental prosthesis. Themold and molded dental prosthesis therein are then cooled to 800° C. ata rate of 10-15° C. per minute. Then they are cooled at a tempering rateof 1° C. per minute from 800 to 600° C. (which avoids the creation oftension at the glass transition temperature). Following this temperingthey are cooled from, 600° to 200° C. at a rate of 10-15° C./per minute.They are then removed from the press system and the mold is divestedfrom the dental prosthesis. The dental prosthesis has a flexuralstrength of 600 MPa when tested according to ISO 6872.

The flexural strength of the dental prosthesis of Example 9 and 10 issurprisingly greater than the dental prosthesis formed in Example 8which has a flexural strength of 300 MPa.

Investment material made as described in Example 11 is used to preparemolds in which the ceramic glass molding composition of Example 8 ismolded. The investment material of Example 11 exhibits a lowercoefficient of thermal expansion than the investment material of Example1.

EXAMPLE 11 INVESTMENT MATERIAL

Investment material is prepared by mixing 1.0 part by weight ofmagnesium oxide, 1.0 part by weight of monoammonium phosphate, and 98.0parts by weight of magnesium stabilized zirconium oxide (ZrO₂ -MgO 94/6,Lonza Gmbh, Germany) powder with particle sizes less than 100 μm andwhich has the following chemical composition:

    ______________________________________                     Percent by Weight    ______________________________________    ZrO.sub.2  and HFO.sub.2                       93.5    MgO                6.0    SiO.sub.2          less than 0.4    CaO                less than 0.2    Al.sub.2 O.sub.3   less than 0.2    Fe.sub.2 O.sub.3   less than 0.1    TiO.sub.2          less than 0.2    ______________________________________

This powder mixture is intimately mixed together and combined with 17 mlof aqueous silica sol (Lewasil 200, 30% Bayer AG, Germany), whichincludes 30 percent by weight of silica. This mixing is done undervacuum for about of 1 minute until a homogeneous mass of investmentmaterial is obtained. The hardened investment material has a coefficientof thermal expansion of 5.0 10⁻⁶ per K and shrinkage upon sintering ofless than 1%. The investment material has a compressive strength ofgreater than 25 MPa.

EXAMPLE 12 VENEERING DENTAL PROSTHESIS

A molded dental bridge made by the procedure of Example 5 has acoefficient of thermal expansion about 6.5×10⁻⁶ per K. A coating ofveneering ceramic powder having the following composition is applied tothe bridge.

    ______________________________________                Percent                by                weight    ______________________________________            SiO.sub.2                  73.8            Al.sub.2 O.sub.3                  10.4            K.sub.2 O                  8.3            Na.sub.2)                  4.0            CaO   1.2            B.sub.2 O.sub.3                  2.3    ______________________________________

The veneering ceramic powder has a median particle size of 18 μm. It ismixed together with suitable pigments and opacifiers to give the desiredtooth shade. The mixture is applied to the surface of the substructurefrom a slurry in water. The coefficient of thermal expansion of theveneering ceramic is 6.7×10⁻⁶ per K and its flexural strength and acidsolubility are 66 MPa and 0.02% respectively determined according to ISOStandard 6872. The veneer powder coated dental bridge is fired at atemperature of 1000° C. to form a veneered dental bridge without theformation of cracks or any other visually detectable evidence offailure.

EXAMPLE 13 VENEERING DENTAL PROSTHESIS

A molded dental bridge made by the procedure of Example 10 has acoefficient of thermal expansion about 8.05×10⁻⁶ per K. A coating ofveneering ceramic powder having the following composition is applied tothe bridge.

    ______________________________________                Percent                by                Weight    ______________________________________            SiO.sub.2                  68.9            Al.sub.2 O.sub.3                  14.5            K.sub.2 O                  8.6            Na.sub.2 O                  4.3            CaO   1.8            Li.sub.2 O                  0.8            Tb.sub.2 O.sub.3                  0.8            CeO.sub.2                  0.2    ______________________________________

The veneering ceramic powder has a particle size of 18 μm and afteraddition of pigment is applied to the surface of the molded dentalbridge as an aqueous slurry. This coated bridge is fired at 1000° C. toform a veneered dental bridge without formation of cracks or othervisually detectable evidence of failure. The coefficient of thermalexpansion of the veneering ceramic is 7.9×10⁻⁶ per K. Its solubility is0.01% in acid and its flexural strength is 78 MPa as determinedaccording to ISO Standard 6872.

                  TABLE 1    ______________________________________    Ceramic-glass compositions and products of Examples 3-10    Example  3      4      5    6    7    8    9    10    ______________________________________    Composition    (Percent by    Weight)    Zirconia 90     80     75   70   60   0    56   42    ZrO.sub.2    Alumina Al.sub.2 O.sub.3             0      0      0    0    0    70   14   28    lanthanum             10     20     25   30   40   30   30   30    aluminoboro    silicate    glass    Physical    property of    product    Flexural 470    550    620  600  300  300  600  600    strength     MPa!    ______________________________________

EXAMPLE 14 INVESTMENT MATERIAL

An investment material is prepared by mixing 100 g of ceramic powderwith particle sizes less than 100 μm, having the following chemicalcomposition:

    ______________________________________                        Percent                        by                        Weight    ______________________________________    Ca-stabilized Zirconia                          50    Quartz                20    Christobalite         9    Fused Silica          15    Magnesium Oxide       2.25    Monoammoniumhydrogenphosphate                          2.25    Magnesium Phosphate   1.50    ______________________________________

with 19 ml of aqueous silica sol containing 35 percent by weight ofsilica. This mixing is done under vacuum for about of 1 minute until ahomogeneous mass of investment material is obtained. This investmentmaterial has a coefficient of thermal expansion of 11×10⁻⁶ per K.

EXAMPLE 15 MOLD MAKING PROCEDURE

A model of wax (wax-up) of the dental prosthesis to be molded isprepared. A mold ring (as shown in FIG. 1 at 40) is placed on a worksurface. A 1 mm thick lining of refractory felt (Kaoliner, DentsplyGmbh) saturated with water is placed within the ring. A portion of newlymixed investment material prepared as described in Example 14 in anamount, sufficient for embedding half the model (wax-up) of the dentalprosthesis to be molded is poured into the mold and allowed to standuntil it hardens. This requires approximately 30 minutes. The surface ofthe hardened investment material is coated with a petroleum jelly as aseparating medium upon which newly mixed investment material is pouredto fill the mold ring and allowed to harden. After approximately 1 hour,the mold is opened. The wax up is physically removed and the cavity ofthe mold is filled with the ceramic/glass powder composition of Example16.

EXAMPLE 16 PREPARATION OF DENTAL PROSTHESIS FROM POWDER COMPOSITION OFZIRCONIA AND GLASS

97 g zirconia (3Y-TZP) is blended with 3 g lanthanum boroaluminosilicateglass of Example 2 and sintered for one hour at 1100° C. After coolingthe sintered material is milled into a powder having a particle size ofless than 10 μm. Then this powder is mixed with 20 g of a glass with thefollowing composition:

    ______________________________________                Percent                by                Weight    ______________________________________            SiO.sub.2                  66.7            Al.sub.2 O.sub.3                  10.5            K.sub.2 O                  8.3            Na.sub.2 O                  7.4            Li.sub.2 O                  1.8            CaO   3.2            BaO   0.9            CeO.sub.2                  0.5            F     0.7    ______________________________________

This ceramic/glass composition is put in a mold (made by the procedureas described in Example 15). The mold with the ceramic/glass compositiontherein is positioned within a press system as described in Example 2and then heated to 120° C. for 1 hour to ensure the evaporation of thewater and then heated to and then held at 1100° C. This temperature ismaintained for 30 minutes while a pressure of 25 MPa is applied to themold to form a molded dental prosthesis. The mold and molded dentalprosthesis therein are then cooled to 800° C. at a rate of 10-15° C. perminute. Then they are cooled at a tempering rate of 1° C. per minutefrom 800° C. to 600° C. (which avoids the creation of tension at theglass transition temperature). Following this tempering they are cooledfrom 600° C. to 200° C. at a rate of 10-15° C./per minute. They are thenremoved from the press system and the mold is divested from the dentalprosthesis. The coefficient of thermal expansion of the moldedcomposition is 10×10⁻⁶ per K.

It should be understood that while the present invention has beendescribed in considerable detail with respect to certain specificembodiments thereof, it should not be considered limited to suchembodiments but may be used in other ways without departure from thespirit of the invention and the scope of the appended claims.

What is claimed is:
 1. A process for manufacturing a non-metallic dental prosthesis comprising a ceramic frame and an esthetic ceramic veneer comprising the steps of:preparing the ceramic frame by press molding a composition comprising from 1 to 50 parts by weight glass particles and from 50 to 99 parts by weight ceramic particles at a molding temperature from 800 to 1300° C.; applying a slurry comprising a dental ceramic veneering composition to the thus prepared ceramic frame; and firing the ceramic frame having the slurry applied thereto at a temperature lower than the molding temperature to produce said dental prosthesis.
 2. The process of claim 1 wherein said ceramic particles are particles having a longest dimension less than 35 microns and said glass particles are particles having a longest dimension less than 100 microns.
 3. The process of claim 1 wherein said glass comprises a silicate or aluminosilicate glass.
 4. The process of claim 1 wherein said glass comprises silicon, aluminum, lanthanum, zirconia, boron, calcium and yttrium.
 5. The process of claim 1 wherein said glass is a lanthanum borosilicate glass.
 6. The process of claim 1 wherein said press-molding is carried out at a pressure of up to about 40 MPa.
 7. The process of claim 1 wherein said ceramic particles are comprised of yttrium stabilized tetragonal zirconia polycrystals and/or alpha alumina.
 8. The process of claim 1 wherein said ceramic particles comprise zirconia and/or alumina.
 9. The process of claim 1 wherein ceramic particles have a melting point at least 1000° C. higher than the softening temperature of the glass particles.
 10. The process of claim 1 wherein said composition is obtained by overcoating ceramic particles comprising at least one of zirconia and alumina with a minor proportion of a first glass and subsequent comminuting to a powder, and by mixing said powder with a second glass powder.
 11. The process of claim 10 wherein said overcoated ceramic particles are comminuted to less than 35 microns and said second glass particles are comminuted to less than 100 microns.
 12. The process according to claim 1, wherein a mold for press-molding is formed by positioning a wax or wax-substitute model of said prosthesis body within a hardenable investment material composition, hardening said investment material to form a mold having a mold cavity and a subsequently filled with said molding composition.
 13. The process of claim 12 wherein said investment material comprises a filler and a binder.
 14. The process of claim 13 wherein said filler comprises a zirconia.
 15. The process of claim 14 wherein said zirconia is calcium or magnesium stabilized zirconia.
 16. The process of claim 13 wherein said binder comprises silica, magnesium oxide, ammonium phosphate and/or magnesium phosphate.
 17. The process of claim 12 wherein the coefficient of thermal expansion of said molding composition is substantially equal to the coefficient of thermal expansion of said investment material.
 18. The process of claim 1 wherein said dental ceramic veneering composition has a melting point at least 50° C. less than the molding temperature.
 19. The process of claim 1 further comprising etching and treating said ceramic frame to allow dental cement to effectively adhere said prosthesis to a dental supporting structure. 